Added some <see> tags

Added some <see> tags to the documentation comments, and added a CDATA
where it was necessary but not used.
tests(System.Numerics.Vectors/tests): vectors tests should work non non en-US culture environment
Fix typo in InitializeNestedTypesMap
Added "readonly" when possible
Fixed inconsistent casing in filename references to match with what's on disk

This makes the solutions build with xbuild on Mono/Linux on a case-sensitive filesystem
Add an .editorconfig file

Just a start. Could help enforcing the Coding Style
https://github.com/Microsoft/dotnet/blob/master/CONTRIBUTING.md#c-coding-style

What is .editorconfig:
http://editorconfig.org/#overview

Plugins for IDEs to support:
http://editorconfig.org/#download
Reduce number of TLS accesses in AllocFreeConcurrentStack<T>
Fix typos in XML comments
Added trim_trailing_whitespace = true
Add optimization for ImmutableList<T>.AddRange when adding non-empty range
to non-empty list
check for null parameter as base class does, but throw ArgumentException
Apply same optimization for ImmutableList<T>.InsertRange, also add more
  test coverage
Merge branch 'fix-vectors-tests' of https://github.com/jbuiss0n/corefx into fix-vectors-tests
Merge branch 'master' of https://github.com/AndreyAkinshin/corefx into remove-null-check
Merge branch 'issue-43' of https://github.com/andi9310/corefx into add-readonly
Merging content between contributing.md and developers.md
Updated .gitignore

 - Visual Studio temporary files, build- and test results
 - Files generated by popular Visual Studio extensions
 - Windows generated- and system files
 - Linux generated- and system files
 - OSX generated- and system files
Merge branch 'typo-md-reader' of https://github.com/jbevain/corefx
XmlAttributeCollection.SetNamedItem(XmlNode) returns null if provided a null parameter
System.Xml.XmlDocument.Tests formatter changes